Method for stimulating hydrocarbon-bearing formations

ABSTRACT

An approved method is disclosed for the stimulation of formations which exhibit a high oil content and porosity, but having a low permeability and therefore poor primary production. The method comprises the placement of explosives at predetermined points within a formation so as to effectively fracture a large portion of the formation subsequent to the explosive placement and detonation. A fracture network is created with interwell communication. A well is drilled into the fracture zone and oil produced therefrom.

United States Patent [151 3,674,089

Moore July 4, 1972 54 METHOD FOR STIMULATING 2,676,662 4/1954 Ritzmann ..l66/299 HYDROCARBON-BEARING 3,464,490 9/1969 Silverman ..166/247 X FORMATIONS FORElGN PATENTS OR APPLICATIONS [721 Rum", 1,278,435 10/1961 France ..166/247 [73] Assignee: Cities Service Oil Company, Tulsa, Okla.

Primary Examiner-Stephen J. Novosad Filed: p 10, 1970 Attorney-J. Richard Geaman [21] App]. No.: 70,975 ABSTRACT An approved method is disclosed for the stimulation of formalll. "1663135511 tions which exhibit a g 0 content and p y, but having [58] Fieid 247 63 a low permeability and therefore poor primary production. 8 4 6 The method comprises the placement of explosives at predetermined points within a formation so as to effectively fracture a large portion of the formation subsequent to the ex- [56] References cued plosive placement and detonation. A fracture network is UNITED STATES PATENTS created with interwell communication. A well is drilled into the fracture zone and oil produced therefrom. 3,342,257 9/1967 Jacobs et a]. ..166/247 3,332,480 7/ 1967 Parrish ..l66/245 11 Claims, 2 Drawing Figures WWW/g PATENTEBJUL 4:912 I 3, 74,0 9

9 Ho W4 V TED R. MOORE,

INVENTOR.

METHOD FOR STIMULATING HYDROCARBON- BEARING F ORMATIONS BACKGROUND or THE INVENTION The present invention is a method for the recovery of oil from low permeability formations. More particularly, the present invention represents a method for the recovery of oil from low permeability formations, for example .chalk formations, by the stimulation of the chalk formation by sequential or simultaneous explosive fracturing with subsequent reentry into the formation for the production of oil therefrom.

In the primary and secondary production of natural gas and petroleum from subterranean reservoirs one often encounters areas of low permeability or homogenous reservoirs'of extremely low permeability which, however, exhibit a high porosity and contain a large amount of oil. By permeability is meant the measure of a reserviors capacity for transmitting fluid through its pores. This capacity is often expressed in petroleum engineering units as darcies or millidarcies. A darcy is that capacity of reservoir rock required to allow the flow on one cubic centimeter of one centipoise viscosity fluid through one centimeter square of rock per second under a pressure of one atmosphere per centimeter. By porosity is meant that volume of the reservoir rock which is void and may contain therein the reservoir fluid. Therefore, a given increase in permeability and porosity will result in the capability of an equivalent increase of flow through the reservoir rock which is desirable for the production of petroleum therefrom. Whenever reservoir rock is plugged or homogeneously exhibits a low permeability, there exists a decrease in the capabilities of a reservoir fluid to flow as it is restricted or reverted to other portions of the reservoir rock matrix or in the homogeneous case, no flow may occur at all. During reservoir stimulation an increase in permeability is established and greater flow of reservoir fluids afforded through the stimulated areas.

The production interval of hydrocarbon containing reservoirs generally comprise strata of varying permeability. However, a particular problem occurs in chalk formations, for example the Anona Chalk found in Louisiana and throughout the southeastern United States. It is a particularly difficult problem to obtain sufficient oil recovery from these chalk formations. lt is necessary to stimulate the chalk formation so as to yield a greater effective drainage radius to the wellbore.'ln the particular example of the Anona Chalk, which is generally found at a depth of 1,500 feet from the earths surface and from 100 to 200 feet in thickness, porosity in the range'of from 25 to 35 percent of the total pore volume is found. The oil in place is generally found to be 1,300 to 1,500 barrels per acre foot. Although the physical properties of the reservoir appear to be conducive for the primary and secondary recovery of oil, these chalk formations have a permeability of about 0.2 millidarcies. Therefore, typical primary production in these areas is only on the order of 5 to 10 percent of the total oil in place.

Many well stimulation techniques have been utilized to enhance production from these chalk formations, including hydraulic fracturing, acidizing, and the implacement of explosives in the wellbore to propagate and horizontally extend fracture therein. In the particular case of hydraulic fracturing, the hydraulically fractured network formed particularly heals rapidly, even with the use of propping agents such that the fractures close and reseal themselves after continued production, or in many cases become eventually clogged with debris or particles of the formation so that production is eventually curtailed. Acidizing is more of a wellbore treatment than a formation treatment in that the effective drainage radius of the wellbore is not significantly increased so that acidizing is a temporary remedial technique for cleansing a wellbore. The implacement of explosives in the wellbore propagate horizontally extended fractures. The distance from the wellbore actually stimulated is generally small. In a chalk or dense, brittle material which does propagate the shock well wave along cleavage plains, stimulation is obtained but is not propagated far enough into the reservoir to yield significant stimulation. Therefore, none of the above methods have been found to be effective for increasing the wellbore radius and to sufficiently increase the productivity of the oil bearing formation. The treatments are expensive and often only of short duration, with the reservoir eventually returning to its original state. Required is a reservoir treatment which will adequately stimulate the reservoir and have lasting stimulation properties such that a majority of the chalk formations will be rendered productive.

It is an object of this invention, therefore, to provide an improved reservoir treatment technique.

It is another object of the present invention to provide a reservoir treatment technique which will simultaneously stimulate and increase the wellbore drainage radius of chalk formations for oil production therefrom.

It is still a further object of the invention to provide a reservoir treatment technique which is both economical and uncomplicated, and which will stimulate the homogeneously non-productive oil bearing formations of a chalk reservoir by either a simultaneous or sequential detonation of explosives therein with subsequent production from the areas in which stimulation has occurred.

With these and other objects in mind, the invention is hereinafter set forth with particular reference to the following description and drawings.

SUMMARY OF THE INVENTION The objects of the present invention are accomplished by a method for stimulating a hydrocarbon bearing fonnation. The method comprises the drilling of one or more wells in the formation. Explosives are implaced in the one or more uncompleted wells and the adjacent formation to these wells. The explosives are subsequently detonated such that the formation is extensively fractured. The uncompleted wells are plugged and abandoned and a production well is drilled and completed within the fractured network of the formation. In general, it is preferred that four uncompleted wells and a fifth completed well be drilled to form a five-spot pattern with the completed well encompassed by the four uncompleted wells. It is preferred that the explosives be detonated simultaneously to further propagate the explosive waves within the fonnation and thereby fonn an extensive fracture network between the uncompleted detonated wells. It is also preferred that the uncompleted wells be hydraulically fractured prior to the implacement of the explosive to further enhance the displacement of the explosive within the formation and thereby further propagate the formation of the fracture network therein.

BRIEF DESCRIPTION OF THE DRAWINGS DETAILED DESCRIPTION OF THE INVENTION The present invention is an improved method for the stimulation of formations. In particular, chalk formations will be dealt with and disclosed herein. The invention is similarly applicable to any formation having a high porosity and oil content with a low permeability and is especially effective when a formation has cleavage planes or the characteristic of being brittle and easily shattered or broken. The invention comprises a method for stimulating the chalk formation by the implacement of explosives at predetermined points within the formation so as to effectively fracture a large portion of the formation. Although the explosives may be sequentially detonated, it is preferred that the explosives be simultaneously detonated such that the amplitudes of the shock waves may build upon one another and further propagate themselves through the reservoir to form a more extensive fracture network within the formation. The fracture network is created and designed so as to produce interwell communication between the uncompleted detonated wells. Subsequent to the explosive placement and detonation, a well is drilled into the fracture zone and oil produced therefrom. The particular novelty of the invention resides in the fact that wells are drilled into the formation but not completed in any manner. An explosive charge of a predetermined capacity is placed within the formation through the uncompleted wellbore and subsequently the well is partially filled with a packing material and tamped. The explosives are detonated with fracture areas created. The uncompleted wellbores are then plugged and abandoned. A re-entry well is drilled into a selected portion of the fracture network to produce the oil. By this method a greater contact and drainage area is provided in the chalk formation, while expensively completed wells need not be set into the formation. In conventional stimulation methods the stimulation is directed towards the stimulated well, however, by the method of the present invention stimulation and well flow are directed away from the stimulated well providing a nonhealing fracture area from which oil may be produced.

To more fully understand the present invention, FIG. 1 is presented in which a five spot has been formed by the drilling of four uncompleted wells 101, 102, 103 and 104, respectively, in which the explosive has been placed and detonated to form a fractured network 105, 106, 107 and 108, respectively, within a formation. The wells are plugged and abandoned and a production well 109 is drilled into the fracture network 110 to form inter-connecting and communicating production well 109 with the fractured areas formed from uncompleted and detonated stimulation wells 101 and 102, 103 and104.

To more exactly understand the present invention, FIG. 2 is presented which depicts a cross-sectional view of the reservoir of interest taken along line 1-1 of FIG. I. Uncompleted wells 204 and 202 are shown. Well 204 consists of wellbore 214 traversing from the earth's surface 222 through overburden rock 219 and into the reservoir of interest 225. As shown in FIG. 2, the explosive has been detonated such that a rubble mass 212 fills the uncompleted well with a fracture network of rubble chalk 208 adjoining the wellbore and an extensive fracture system 210 communicating between the wells 204 and 202. Well 204 is then plugged by close packer 213 and cement plug 215 at the earths surface such that it is plugged and abandoned. Similarly, well 202, consisting of wellbore 217, has a rubble mass 21 l therein its wellbore 217 with a fracture network 206 adjacent thereto and the extensive fracture system 210 connecting it with the other wells in the vicinity. The well is isolated by use of close packer 216 and cement plug 218 and thereby plugged and abandoned. Subsequent to the drilling and detonating of an explosive in the uncompleted wells 204 and 202, production well 209 is drilled and completed consisting of casing 220 and tubing string 221 contained therein extending from the earths surface 222, through the overburden rock 219 and into the reservoir of interest 225. Perforated wellbore liner 223 is also provided through which hydrocarbon fluids may flow. The tubing string 221, which subsequently becomes the production string, is isolated from the casing and overburden rock by isolation means, for example packer 224, such that the wellbore liner 223 interconnects and communicates with the extensive fracture network 210 previously created such that subsequent production from production well 209 is greatly enhanced with a minimal expenditure of equipment.

The stimulation wells are drilled in the formation but not completed in any manner and an explosive charge of a predetermined capacity is placed within the formation and subsequently detonated. It is preferred that the wellbore be filled with packing material and tamped prior to the detonation of the explosive. Thereupon, either simultaneously or sequentially, the explosives may be detonated in the fractured area created. As disclosed, it is particularly preferred that the explosives be detonated simultaneously such that the explosive shock waves and amplitudes thereof may build one upon the other in order that a greater force be exerted upon the formation, and in the particular case of the chalk formation or otherbrittle formations will thereby rupture the formation and create a more extensive fracture network. Thereafier, the uncompleted wellbore, as indicated, may be plugged by the introduction of cement or a similar sealing material and close packers or other conventional techniques. The re-entry or production well is then drilled within the fractured network.

Although the five-spot pattern has been disclosed herein, it is, of course, within the scope of the invention disclosed that any well pattern may be utilized withany number of production wells thereafter introduced within the fracture network. It is also preferred that the formation be hydraulically fractured prior to the introduction of the explosive within the fonnation, and in particular by the use of a propping agent, so as to cause a greater area] extent and wave propagation area upon the detonation of the explosive and thereby form a more extensive drainage area for the production wells thereafter drilled and completed. Standard propping materials which may be used in conjunction with hydraulic fracturing steps such as those selected from the group consisting of nut shells, glass beads,

metal pellets, gravel and sand. It is preferred that non-autodetonating explosives be utilized, although typical conventional explosives which may be used in the present invention do include the tri-nitrotoluene and tri-nitroglycerin explosives generally used for in situ fracturing of formations. Explosives of a wide variety may be utilized to effectuate the large fracture area desired such as those disclosed in Perry, et al., US. Pat. No. 3,378,416, may be used in conjunction with the present invention such that a greater fracture area may be initiated at a low economical cost for greater production and economic feasibility.

The present invention, therefore, represents a significant advance in the art of well stimulation for the primary and secondary recoveryof hydrocarbons. The invention may be utilized in combination with any production techniquecommonly utilized to enhance hydrocarbon production and provides a relatively simple and inexpensive method of treatment. The invention avoids the heretofore existing complications of combating a low reservoir permeability, especially that in chalk formations where the healing characteristics of fracture created therein may cause extensive problems to the production of oil. It will be appreciated by those in the art that other embodiments of the present invention exist without departing from the scope of the invention as presented.

Therefore, I claim:

1. A method for stimulating a hydrocarbon bearing formation comprising: 1

a. drilling one or more uncompleted wells into the formation;

b. placing chemical explosive as the only explosive in the uncompleted wells and adjacent formation;

c. detonating the chemical explosives;

d. plugging and abandoning the uncompleted wells; and

e. drilling and completing a production well within the fracture network formed.

2; The method of claim 1 in which the formation is chalk.

3. The method of claim 1 in which the chemical explosives are detonated simultaneously.

4. The method of claim 3 in which the chemical explosives are non-auto-detonating.

5. The method of claim 1 in which four uncompleted wells and a fifth completed well are drilled to form a five-spot pattern with the completed well encompassed by the four uncompleted wells.

6. The method of claim 5 in which the formation is chalk.

7. The method of claim 6 in which the chemical explosives are detonated simultaneously.

tern with the completed well encompassed by the four uncompleted wells.

11. The method of claim 1 further comprising partially filling the uncompleted well with a packing material prior to detonating the explosive placed therein.

* i it i 

1. A method for stimulating a hydrocarbon bearing formation comprising: a. drilling one or more uncompleted wells into the formation; b. placing chemical explosive as the only explosive in the uncompleted wells and adjacent formation; c. detonating the chemical explosives; d. plugging and abandoning the uncompleted wells; and e. drilling and completing a production well within the fracture network formed.
 2. The method of claim 1 in which the formation is chalk.
 3. The method of claim 1 in which the chemical explosives are detonated simultaneously.
 4. The method of claim 3 in which the chemical explosives are non-auto-detonating.
 5. The method of claim 1 in which four uncompleted wells and a fifth completed well are drilled to form a five-spot pattern with the completed well encompassed by the four uncompleted wells.
 6. The method of claim 5 in which the formation is chalk.
 7. The method of claim 6 in which the chemical explosives are detonated simultaneously.
 8. The method of claim 7 in which the chemical explosives are non-auto-detonating.
 9. The method of claim 1 further comprising hydraulically fracturing the uncompleted wells prior to the implacement of the explosive.
 10. The method of claim 9 in which four uncompleted wells and a fifth completed well are drilled to form a five-spot pattern with the completed well encompassed by the four uncompleted wells.
 11. The method of claim 1 further comprising partially filling the uncompleted well with a packing material prior to detonating the explosive placed therein. 